Quest for Economic Sources of Renewable Energy
The development of a practical wave energy converter has been the focus of attention from a number of engineers and theoreticians over the past twenty five years. Theoretical understanding of sea waves and technical expertise in related marine engineering has gained immeasurably from the offshore oil and gas industries during the same period. Growing concern with global climate change has led to an increased sense of urgency in the quest for commercially viable renewable energy sources.
The Size of the Wave Energy Resource
The potential of wave energy has been recognised for many years. The size of this resource has been estimated to be 219 gigawatts along the coasts of the European Union, or more than 180 terawatt hours each year. The Wave power off the west coasts of Ireland and Scotland, where the winter resource is approximately twice that available during summer months, ranks with the highest levels in the World.
The Offshore Resource is Greater
Wave energy is lost by friction with the sea bottom as the sea becomes shallow (water depths of half a wavelength or less). This is most pronounced where wavelengths tend to be long, as off the NW coast of Europe. On or close to the shore the availability of this already attenuated resource is greatly diminished by the lack of physically suitable sites and restrictions imposed by planning controls.
Development of Wave Energy Converters.
Research and development into wave energy converters (WECs) over the past twenty-five years, plus the knowledge and practical experience gained from the offshore oil and gas industries, has now reached a stage where robust and effective wave energy converters with installed capacities of one megawatt and greater are being developed.
Categories of Wave Energy Converters
The wave energy resource may be split into three broad categories, based on where the energy from waves may be recovered:    1. in the open sea, i.e. offshore    2. on or close to the shore line, i.e. on-shore or inshore    3. outside the normal area of breaking waves but not in the deep ocean, i.e. near shore.
The very large number of devices and concepts proposed to date has been classified and described in summary form for the Engineering Committee on Oceanic Resources by the Working Group on Wave Energy Conversion (ECOR draft report, April 1998). This follows a similar classification based on the intended location, i.e. offshore, near shore to offshore, and on-shore.
Wave Energy Converters (WECs) may also be classified in different ways according to their operating principle and the ways in which they react with waves. In terms of practical application, only a very few types of device are presently, or in the recent past have been, in use or under test.
A significant fraction of the present generation of WEC devices incorporate an Oscillating Water Column (OWC). OWC devices are typically those where the wave is confined in a vertical tube or a larger chamber and, as it surges back and forth, drives air through a power conversion device typically an air-turbine. Megawatt-scale OWC devices are now at an advanced stage of development. One such device, built in a rocky gully on the western shore of Pico in the Azores, is a reinforced concrete chamber partly open at one side below the waterline to the action of the waves. A similar but slightly smaller device, the LIMPET, has been installed on the cliff face of Islay in Scotland. These two installations would seem to be the best-developed and perfected WEC systems of this size currently available. It is, however, unlikely that any one such installation will have an installed capacity greater than two megawatts and the number of suitable sites has to be extremely limited.
The present invention relates to an apparatus that may be of at least a comparable size, and capable of being deployed offshore and in large arrays. It is of a class of WEC's known as Point Absorbers.
Point Absorbers
Point absorbers are usually axi-symmetric about a vertical axis, and by definition their dimensions are small with respect to the wavelength of the predominant wave. The devices usually operate in a vertical mode, often referred to as ‘heave’. Typically, a surface-piercing float rises and falls with the passing waves and reacts against the seabed or a taut mooring. As such they are capable of absorbing energy arising from changes in the surface level rather than from forward motion of breaking seas. The theoretical limit for the energy that can be absorbed by a single isolated, heaving, axi-symmetrical point absorber has been shown to depend on the wavelength of the incident waves rather than the cross sectional area of the device, i.e. from the wavelength divided by 2π. Thus the wavelength is a critically important criterion, resulting in the attraction of locating the point absorber devices well outside the region of breaking waves, and where they will be open to long wavelength ocean swell or ‘heave’.
Point absorbers may react against the seabed (therefore necessarily sited in relatively shallow water, usually near-shore), or be floating and react against the inherent inertia of one of its components.
Small-scale practical point absorbers such as fog horns and navigation buoys, both of which may incorporate OWCs, have been in use for many years. Typically these have a power of a few hundred watts.
Self-Reacting Heaving Buoy Point Absorbers.
There have been several attempts to develop wave energy converters based on the self-reacting heaving buoy principle. One such example is a heaving buoy which reacts against an inertial plate suspended below. This concept has been described and analysed by Berggren, L. and Johansson, M., Hydrodynamic coefficients of a wave energy device consisting of a buoy and a submerged plate. Applied Ocean Research, 0141-1187/92/05.00 and by Falnes, J., Wave-energy conversion through relative motion between two single-mode oscillating bodies (OMAE, Lisbon, Portugal, Jul. 5-9, 1998).
A second variation of the heaving buoy principle is described in an international patent application, WO 97/41349. In this, a single heaving buoy reacts against a column of water trapped in a cylinder suspended vertically below and open at either end, by means of a wide piston moving reciprocally within the cylinder. The column of water moved by the piston acts as an inertial mass; this arrangement is known as an accelerator tube. Similar technology is known and described in U.S. Pat. No. 4,773,221.
In these illustrative examples and all such self-reacting heaving buoy systems, there are essentially three basic components: a heaving buoy on the surface, some form of reaction device suspended below (an inertial plate, accelerator tube, etc.) and a load resistance or power take-off placed between them.
Latching and Phase Control
It is also known to use a principle of latching the phase control of a heavy body The principle of latching a heaving (vertically oscillating) body in irregular waves having been described by Budal and Falnes in 1978 British Patent No. GB 1587344.
Their idea was to force the phase of a heaving float to follow that of the waves, which had a significantly lower natural frequency (longer period). In this way greatly amplified motions and correspondingly larger power levels were achieved.
They disclose the holding of the heaving body at the top or bottom of its cycle by a hydraulically operated latching mechanism (functioning as a parking brake), locking the heaving float to a long rod attached to the bottom of the wave channel. It was then released so that it would resume motion in direction and in phase with the wave. Further theoretical analysis has been completed by various researchers. Two forms of such ‘phase control’ are now recognised, i.e. latching as described and continuous control which may be applied throughout the cycle and may involve power being returned to the heaving device.
Variable Buoyancy Apparatus
A further development in self-reacting point absorbers incorporates a three-body point absorber comprising a surface float, a submerged variable buoyancy and an inertial mass. Such a device is known and described in our corresponding international application WO 99/28623. Such a device does not provide an optimum transfer of energy from the passing waves to the converter.
Therefore is therefore a need for an improved wave energy conversion device.